This invention relates to improved methods and apparatus for evaluating the condition of subsurface oilfield piping and, more particularly, relates to apparatus for nondestructive testing of the state of deterioration of relatively small diameter subsurface tubing and casing.
It is common knowledge in the petroleum industry to use steel or iron pipe in nearly all oil and gas wells. Such piping serves several useful purposes including shutting off water bearing formations, preventing formation deterioration and shutting off intermediate oil or gas zones when it is desired to drill deeper. During the lifetime of a well a variety of conditions may result in the deterioration of such subsurface piping, including pits, cracks, holes, thin walls, structure changes in the metal and the like. Such deterioration may result from various causes. Electrochemical theory postulates a tendency of steel, or other materials, in an electrolytes environment, such as subsurface formations, to go into solution causing corrosive deterioration. Likewise, during drilling operations drill pipe collars may rub on the inside wall of the pipe causing excessive wear.
There have been various proposals for the design of equipment to measure pipe anomalies while the piping is still in place. One such system which has been widely accepted by the industry is described in U.S. Pat. No. 3,543,144, issued to W. T. Walters et al on Nov. 24, 1970. This system includes an elongated magnetizer assembly including a pair of elongated pole pieces, having a diameter only slightly less than the inside diameter of the piping, and a central core having a magnetizing winding thereon between the pole pieces. A plurality of detectors are positioned between the pole pieces into contact with the interior surface of the casing. A high-intensity unidirectional magnetic field emits from the magnetizing winding with the object of saturating the ferrous piping. If there is no defect in the piping, the magnetic flux lines pass through the piping between the pole pieces. When a defect in the casing exists a portion of the magnetic field will "leak" out of the piping and flow about the defect. This flow is detected within the wall contact detectors.
Yet another system for inspecting oilfield piping is illustrated in U.S. Pat. No. 3,940,689, issued to W. M. Johnson, Jr. on Feb. 24, 1976. This system includes an elongated body member having longitudinally spaced pole pieces magnetically coupled to the ends of an elongated reduced diameter core of magnetic material having a magnetic permeability no greater than the permeability of the pipe to be inspected and a transverse cross sectional area less than the known pipe metal area. A coil wound on the core between the pole pieces generates a unidirectional magnetic flux field and two groups of detectors longitudinally separated on the body member detect flux leakage variations.
While both systems described above have found acceptance in nondestructive testing of subsurface piping they are limited to use in pipe generally in excess of 4.5 inches outside diameter. Accordingly, the present invention provides methods and apparatus for nondestructive testing of the state of deterioration of relatively small diameter subsurface tubing and casing, typically having an outside diameter less than 4.5 inches.